An integrated microsystem may be produced using silicon-germanium compounds and germanium, as referred to in Franke, A. E., Jiao, Y., Wu, T., King, T.-J., and Howe, R. T., Post-CMOS modular integration of poly-SiGe microstructures using poly-Ge sacrificial layers, Solid-State Sensor and Actuator Workshop, Hilton Head, S. C., June 2000, pp. 18-21, and in U.S. Pat. No. 6,210,988.
Integrated Microsystems are electronic systems or a combination of electronic and mechanical systems, such as for example resonators, accelerometers or rotational speed sensors.
To produce them, a conductor layer of silicon-germanium or aluminum is first applied and structured on a wafer having electronic circuits, through electronic passivation. There is a diffusion barrier on the aluminum, of titanium nitride (TiN) for example, to prevent diffusion between the aluminum conductor layer and a SiGe layer. Without this barrier, aluminum atoms would diffuse into the SiGe layer and in some circumstances, would change the material properties of the SiGe layer so that its favorable structuring properties and its good mechanical properties are degraded.
To accomplish the structuring, a layer of photoresist is applied to the layer of silicon-germanium or aluminum-(TiN), and is then exposed. The exposure defines the places where the previously applied photoresist will remain, the exposure phase being followed by a development phase. Next the wafer, i.e., the layer of silicon-germanium or aluminum-(TiN), is etched in an etching procedure, the non-masked parts, i.e., the parts not passivated with the exposed and developed resist, being stripped away during the etching process.
On the silicon-germanium or aluminum layer, which constitutes for example a connection between electronic and mechanical components of a microsystem, a sacrificial layer made up for example of germanium or germanium-rich silicon-germanium is usually deposited and structured, the proportion of germanium in the latter material, which may be 80%.
On top of this sacrificial layer the actual functional layer of SiGe is applied and structured. Before the SiGe functional layer is applied, the sacrificial layer may be structured, for example using a reactive plasma. The SiGe functional layer has a smaller proportion of germanium than the sacrificial layer; a germanium proportion of the SiGe functional layer smaller than 80% may be provided, for example. Over a germanium-rich SiGe sacrificial layer or a germanium sacrificial layer, a SiGe functional layer having a smaller proportion of germanium is thus provided, which is structured into the geometry of the sensor elements using available RIE methods.
After the application of this SiGe functional layer, the sacrificial layer is at least partially removed using an oxidizing agent, there typically being areas located under the sacrificial layer having passivation of electronic circuits as well as open bonding pads and vias, possibly even open conductors, made up as a rule of aluminum, aluminum-silicon or aluminum-silicon-copper. During the sacrificial layer etching these metallic regions are exposed, so that the metallic regions come into direct contact with the etching solution and thus are able to interact with it.
Hydrogen peroxide may be used for example as the etching solution, as referred to in German Patent 38 74 411. It does not attack the electronic passivation, so that no special precautions are necessary to protect the passivation.
A disadvantage of this method, however, may be that while the sacrificial layers are being etched, a reaction occurs between the etching solution, for example hydrogen peroxide, and any open conductors and bonding contacts of aluminum or aluminum alloys, which in some cases even results in complete destruction of bonding pads and other open metallic areas before the sacrificial layer to be stripped is completely removed. The damage to or destruction of the bonding pads or conductors may be comparable in certain circumstances to destruction of the entire integrated microsystem. The etching attack results from the fact that during dissolution of the Ge sacrificial layer or SiGe sacrificial layer, acidic reaction products are formed which lower the pH value of the H2O2 solution so greatly and shift it so far into the acidic range that the aforementioned metallic structures are also attacked. An approximately neutral H2O2 solution does not result in the unwanted etching attack on the metallic structures.
To prevent such an attack on or such destruction of the bonding pads or conductors of a microsystem, in practice an attempt is made to provide the bonding pads or conductors with passivating layers. However, this may necessitate additional process steps, which lead to an increase in the manufacturing costs.